The present invention relates to an apparatus for use in mooring a floating structure, such as a TLP or VMP, to the floor of a body of water and, more particularly, to such an apparatus for anchoring an upper end of a tension member within the floating structure.
2. Setting of the Invention
A floating structure used for drilling for and/or for production of hydrocarbons, such as tension leg platform (TLP) or a vertically moored platform (VMP), includes at least one working deck supported above the waterline by a plurality of vertical columns which extend into the water. A plurality of pontoons are horizontally connected between the columns beneath the waterline to provide additional buoyancy and structural integrity to the floating structure. The floating structure is anchored to the floor of a body of water by connecting a plurality of tension members, also known as tendons or tethers, from an anchoring template on the floor of the body of water to a connection point within the floating structure. An upper portion of each of the tension members is connected within the column above the waterline at a connection point, called a mooring flat.
Problems have been encountered with this arrangement because the tension members are constantly under static tension and the dynamic tension on each tension member varies in magnitude according to the wind and wave action acting upon the floating structure. A top portion of each tension member includes a locking mechanism which acts upon the mooring flat within the column of the floating structure. What happens is the tension in the tension member puts compressive downward force on the column that could cause the buckling of the column's wall.
To alleviate this problem, additional reinforcement must be added within the column and at other internal supports within the floating structure; however, these additional reinforcements cause a large increase in the weight and fabrication costs of the floating structure.
It is recognized that it is advisable to transfer the mooring forces of the tension members to the lowestmost position within the column. In this manner the tension forces are applied to the strongest portion of the floating structure, allowing for the upper portion of the column to not be unnecessarily placed under the compressive loads and the additional reinforcement is not needed. One arrangement to solve the buckling problem is to place the locking mechanism on a support arm extending from a horizontal pontoon or a vertical column beneath the waterline. However, this arrangement is not desired because the locking mechanism is exposed to water-induced corrosion and is extremely difficult to work upon and to inspect, if necessary. Another arrangement is to place the locking mechanism within a small compartment within a lower portion of the column or the pontoon. However, since this arrangement is again below the waterline, a costly air pressurization system is required to prevent the compartment from being flooded when the locking mechanism is to be worked upon or inspected.
There is a need for an anchoring arrangement for a tension member that is above the waterline for ease of maintenance, but which applies the mooring forces to the lowermost portion of the floating structure to thereby reduce the cost and the weight of the floating structure.